How can I detect diagram region and extract (crop) it from a research paper
I will consider that you need to extract(crop) the digram from the pdf research paper. You can use PyPDF2 or PyMuPDF to extract the images from the PDF file and then you can apply machine learning to do recognition and classification of the images. There are different types of machine learning solutions for image classification and you can start with Convolutional Neural Network and you can start here.

For more information for Recognition and classification of figures  see these  and to extract of figures from scholarly documents read this